Detailed chemical knowlege of the mechanism of interconversion of various nucleotides has progressed to a point at which questions concerning the energetics of the reactions may be answered. This study is designed to explore the energy changes occurring during thymidylate synthesis form the precursor, uridylate. The enzymes involved in this conversion are one of the few successful targets for tumor chemo therapy. Methotrexate, a folate analog competes with dihydrofolate thus preventing cell replication. The purpose of this study is to provide detailed thermodynamic information, about this enzyme system, which may contribute to the developmental compounds of increased efficacy in chemo therapy. BIBLIOGRAPHIC REFERENCES: I.M. Klotz, D.E. Darnall, N.R. Langerman, "Quaternary Structure of Proteins", The Proteins, 3rd Ed. (1975) 293. Mangold, A. and N. Langerman, "The Enthalpy of Oxidation of Flavin Mononucleotide. II. Temperature Dependence of in vitro Bacterial Luciferase Bioluminescence", Arch. Biochem. Biophys. 169:126 (1975).